1. Field of the Invention
The present invention relates to monitoring the properties of a surface and more particularly to the use of second-order nonlinear optics to generate a surface specific position sensitive image of contaminants that are present on the surface
2. Description of the Related Art
In the most general sense, the term xe2x80x9csurfacexe2x80x9d applies to any interface between two media. It is not restricted by the material phase of matter and can be between a gas and a solid, a gas and a liquid, between two liquids, between a solid and a liquid, between two solids, etc.
In the study and characterization of surfaces or interfacial media, it is important to create an image or map of the interface which correlates a property to a location on the surface. Depending on the application, chemical, mechanical or molecular properties may need to be monitored on a surface. Additionally, the properties of a location or locations on a surface need to be monitored simultaneously. This monitor should be nonintrusive, noninvasive and should not be restricted in use by the ambient environment which surrounds it. Consequently, one usually uses optical sensing, as light propagation is not restricted by ambient environment, so long as the environment is transparent to the light. Unfortunately, linear optical diagnostics, i.e., those in which the output frequency is the same as the input, are not surface specific. Output signals will include large contributions from the bulk, so that subtle changes on the surface will only weakly affect the output signal.
There are many techniques which can be used to analyze molecular properties or behavioral properties at an interface. Unfortunately, the vast majority of them have rather severe environmental limitations on their application. Many of them are restricted to ultra-high vacuum, with the implication that any liquid or liquid/solid interface may not be tested. Even if one wanted to inspect a vacuum-solid interface, one would have to take the material to be examined and place it in a high vacuum chamber. This can often be slow, expensive or impossible depending on the size of the particular material in question.
Other surface diagnostic techniques require that the interface be inserted into other environments, such as the interface between two solids, in which case it may be necessary to destroy the particular material to be studied. Still, other techniques require either fabrication of very exotic detection means or require signal integration times which makes them unusable in any sort of real time industrial scenario.
Discussed herein are NLO techniques to solve pragmatic problems that occur in industry. For example, materials processing, surface contamination, surface corrosion, etc. are problems that are addressed. In nonlinear optics, outputs are produced at sum, difference or harmonic frequencies of the input(s). Using second order nonlinear optical surface spectroscopy to examine the physical properties and behavior of a surface or interface was originally proposed in the 1960""s in xe2x80x9cLight Waves at the Boundary of Nonlinear Mediaxe2x80x9d by Bloembergen and P. S. Pershan, The Physical Review, 128, Page 193 (1962). Experimental work involving second harmonic generation was also performed. However, because lasers at the time were comparatively feeble, impractical, slow, etc., there was little subsequent work done on the development of second harmonic generation or, more generally, second order nonlinear optical (NLO) processes at surfaces until considerably later when high peak power lasers became reliable and commercially available.
Recently, researchers have reviewed NLO processing and concluded that lasers had developed enough that they could be used for studying the physical and chemical properties of surfaces and interfaces. For example, a theoretical study of the physics of the interface, and not its engineering aspects, has been performed. See Journal of Vacuum Science and Technology B, Volume 3, Number 5, September October 1985, Pages 1464-1466, Y. R. Shen, xe2x80x9cSurface Studies by Optical Second Harmonic Generation: an Overview.xe2x80x9d
In U.S. Pat. No. 5,294,289, T. F. Heinz et al. discuss the use of second harmonic generation as a means to monitor the epitaxial growth of silicon semiconductor structures in a high vacuum chamber. Specifically, they examined the spectroscopic response at the interface between the electronically active silicon and the insulative layer of calcium. By monitoring the magnitude of the resonance, they could ascertain whether the insulator was present on the surface and whether it had electronically binged to the underlying semiconductor. The system that is used only examines the total intensity of the light that is collected and there is no attempt to associate position sensitive information with the second harmonic. There is also no discussion of the use of second harmonic generation (SHG) for the detection of contamination. In addition, the preferred embodiment only discusses an optical system which employs nanosecond duration pulses.
In U.S. Pat. No. 5,623,341, J. H. Hunt discusses the use of sum-frequency generation for the detection of contamination and corrosion on engine parts. In this incarnation, one of the inputs is a tunable IR beam that is tuned to a resonance of the contamination on the surface. The efficiency of the sum-frequency process is increased (so-called resonant enhancement) when the IR beam is resonant with a contaminant. If the contaminant is not present, there is no resonant enhancement. By comparing on and off resonant signals, the presence and level of contaminant can be deduced. However, there is no attempt to associate position sensitive information with the sum-frequency signal and there is no discussion of the use of laser pulses with femtosecond durations.
In U.S. Pat. No. 5,875,029, P. C. Jann et al. describe a versatile optical inspection instrument and method to inspect magnetic disk surfaces for surface defects. The device provides surface position information of the defects. However, the technique involves only linear optical processes. That is, the input and output light wavelengths are the same.
In U.S. Pat. No. 5,883,714, Jann et al. describe a versatile optical inspection instrument and method to inspect magnetic disk surfaces for surface defects. The device is based on interferometric measurement and detects contaminants by measuring the Doppler shift in the light that results from scanning the light onto a contaminant or defect. By scanning, the device provides surface position information of the defects. However, the technique involves only linear optical processes and senses only phase changes. That is, the input and output light wavelengths are the same.
In U.S. Pat. No. 5,898,499, J. L. Pressesky discusses a system for detecting local surface discontinuities in magnetic storage discs. The device is an interferometric detector which scans the disc in a spiral motion. Local defects cause local changes in phase which are measured by interferometric techniques. This is a linear optical technique.
In U.S. Pat. No. 5,932,423, T. Sawatari et al. discuss a scafterometer for detecting surface defects in semiconductor wafers. This device is a linear interferometric device.
In U.S. Pat. No. 5,973,778, J. H. Hunt discusses the use of second harmonic generation for investigating molecular alignment within a thin polyimide film. The technique uses changes in the second harmonic polarization to determine surface molecular alignment. There is no discussion of semiconductor materials or contamination and there is no attempt to associate position sensitive information with the second-harmonic signal and there is no discussion of the use of femtosecond pulses.
In U.S. Pat. No. 6,317,514 B1, S. Reinhorn et al. discuss a method and apparatus for inspecting a wafer surface to detect the presence of conductive material on the wafer. The device uses UV initiated electron emission to determine the location of conductive areas. Those areas which are metal will emit electrons. If the area which is supposed to be conductive is not, there will be no electron emission.
In U.S. Pat. No. 6,359,451 B1, G. N. Wallmark discusses a system for testing for opens and shorts between conductor traces on a circuit board. The technique uses electron scattering to perform its diagnostics and has no optics associated with it.
In a broad aspect, the surface specific optical diagnostic system includes a first optical source for providing a first laser input directable to an area of a surface to be interrogated. The first laser input has a pulse having a duration of about 50-100 femtoseconds. A second optical source provides a second laser input directable to that surface area. The second laser input also has a pulse having a duration of about 50-100 femtoseconds. These lasers are alignable so that their surface areas of optical illumination overlap on the interrogated surface. The second optical source includes an optical delay line for temporarily synchronizing the first and second laser inputs. An output wavelength discriminator receives reflections of the first and second laser inputs from the interrogated surface. The output wavelength discriminator is substantially non-transmissive at the frequencies of the first laser input and the second laser input, but is substantially transmissive at the sum-frequency of the first laser input and the second laser input. Imaging optics receive an output of the wavelength discriminator and direct the propagation of the output so that a real image is formed after propagation through the imaging optics. A position sensitive detector monitors the intensity of the real image at the sum-frequency so that the sum-frequency, as a function of surface position, is converted to an electronic signal equivalent to the real image. Utilization of the femtosecond duration laser inputs generates enhanced peak optical powers on the interrogated surface, producing an optimized sum-frequency reflection, resulting in minimized interrogation times.
Active imaging, in which one directs a laser at an object and collects the returned light, has been used to create optical representations of targets for some time. Optical techniques have certain advantages over other sensing devices in that they are not restricted by environment and can be used in a remote configuration. Second order nonlinear optical techniques have been used to perform surface sensitive diagnostics, but to the present, have only been used on a point by point examination. These nonlinear optical effects are coherent. That is, the light that is generated by them travels in a well-defined matter, akin to a specular linear reflection. Consequently, it is possible to perform active imaging of a surface, exploiting the nonlinear optical response of the surface. This provides a means to do surface sensitive imaging while maintaining the advantages of an optical technique. When the basic development work was being performed, the laser systems which were employed were typically of nanosecond duration. This duration was not due to system optimization, but was merely based on the lasers that were available at the time. Work performed in these experiments showed that shorter pulses allowed higher peak intensities without causing surface modifications or damage. In order to produce an image, much of the optical intensity is distributed over the area to be imaged, reducing the peak intensity. The incorporation of femtosecond pulses increases the peak intensity to levels which allow for the rapid acquisition of image data.
Other objects, advantages, and novel features will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.